If they're not being damaged in the transplant, it could be any of a number of things - eg, transplanting too early, not enough light, not enough warmth, too much moisture. Or even a combination of things. More information would help. What are you repotting? How much moisture, light and warmth are they getting?

Kevc, seedlings are like new born babies they need TLC, too much of everything can kill them. Too much heat, cold, water, too rich a compost and in some cases unsterilised compost. I use my own mix of seed compost 1/3 compost 1/3 fine grit 1/3 washed sand. The potting on compost is only 1/2 compost 1/4 grit and 1/4 washed sand. An old fork to get under the seedling and lift with plenty of compost attached to the root system, with my potting on tray ready filled with compost and firmed down just a little (do not compact it) poke a hole in the compost with pencil or prodder and ease the seedling into the hole but sink it so part of the stem is also in the hole then gently firm the seedling in. Water trays or pots from the bottom up (stand in a bowl of water until the top of the compost looks just damp) stand aside to drain excess water and then put in a light warm place out of direct sunlight at first and out of cold air flow. Turn the trays daily so they grow evenly and try to make sure the warmth is fairly constant at first, they do not like big variations in temperature.It all sounds a bit time consuming but you get into a rhythm when potting on a lot of seedlings, I put my music on, brain out of gear use a high stool to sit at my compost filling tray dreaming of a lotto win, time flies.One thing to remember some seedlings will fail and you will not need all you manage to strike, discipline is needed to just pot on what you need plus a spare tray then the rest onto the compost heap.Hope this helps,

thanks to everyone who as replied the plants are in the greenhouse I have just found out not to put my pots on the floor which I did and left them over night we had lots off rain at night where the rain water came into my greenhouse the young plant must off absorbed the rain water and that as killed them off going to put a floes floor in to keep the plats off the floor thanks again

If they were on the floor and water was getting into the greenhouse, they may also have got cold - young seedlings do not like this at all. As evryone else has said, warmth, moisture not wetness, light - lots of light, were they getting that at floor level? I know that greenhouses need elastic walls at this time of year, it gets difficult to give everything ideal conditions. I tend to keep older larger plants on the floor (my g/h is paved all through) and the upper warmer areas for young things.